The dawn of a new age: Supersizing offshore wind turbines

Updated: Sep 15

In 2015, Ørsted's 210 MW Westermost Rough offshore wind farm started operating within UK waters utilizing 6 MW wind turbines. Ten years on, the industry will be constructing 1,500 MW offshore wind farms with the next generation of 15 MW wind turbines. In this #RUKGOW21 blog, Simon Cox, Offshore Wind Business Manager at DNV, explains how this growth represents amazing innovation in the design, manufacturing, control and maintenance of turbine technology, but raises questions on the technical and commercial risks of 10-15 MW+ turbines.

Evolution instead of revolution

In recent years, wind turbine manufacturers have adopted an evolutionary approach, leveraging their operational track record to manage the risks in delivering new, larger offshore turbine models. The gradual increase in the rated capacity is seen in the current MHI Vestas and Siemens Gamesa turbines between 6 MW to 10 MW. This ‘uprating’ approach has been achieved through incremental improvements in the design, components and monitoring of the generator, and drivetrain based on experience gained from testing and operating smaller turbine variants.

The length of wind turbine blades is the second area of evolution with increases in rotor diameters from 154m to 174m seen on current 6-10 MW turbines. Longer blades enable greater energy capture, increase annual energy production and provide loads innovation, such as the development of individual blade pitch control systems for rotor diameters in excess of 150m.

This wind turbine evolution has enabled 6-10 MW models to be introduced with relatively low risk, while maintaining investor confidence and enabling low cost of project finance, culminating in projects currently under construction achieving a levelized cost of energy of 50 EUR/MWh compared to 250 EUR/MWh for operational projects in 2015.

New turbines increase risk

The latest increase in rated power of supersized offshore wind turbines has been achieved by increasing the diameter while overcoming the challenge of maintaining a lightweight turbine nacelle. A light nacelle helps to reduce turbine costs and enables installation using current vessels.

Supersize turbines are implementing rotors of over 220m and require blades longer than a football pitch (currently 108m long from blade root to tip). These blades will be lightweight and include an increased volume of carbon fibre incorporated into the composite structure to improve strength. Longer blades with increased loads on the turbine structure need to be managed through innovation in the design and control of the wind turbine, whilst having loads validated through testing. An example of wind turbine control innovation is the implementation of the individual blade pitch control approach which has reduced loads of the current 6-10 MW turbines.

In 2019, Siemens Gamesa and GE Wind Energy installed prototype 10-14 MW turbines in Europe to validate their design and performance. Full scale turbine testing is key to validate new turbine models. Accelerated life testing of the main wind turbine components, including the blades, pitch system, hub, drivetrain and generator, provides reassurance that the wind turbine will operate efficiently over its lifetime.

It is expected that turbine manufacturers will provide robust comprehensive warranties to de-risk these new machines. This will provide stakeholders with assurances on technical availability and performance in the early phases of commercial projects, whilst enabling manufacturers to demonstrate confidence in their products.

The offshore wind supply chain needs to evolve to support new wind turbines coming in 2022/23 and deliver further cost reduction. For example, turbines will require larger, heavier foundations to support them, and the supply chain is ready to produce XXL monopile foundations for 10-15 MW turbines. The foundation supply chain is capable of producing monopile foundations of up to 2,000 tons with a diameter in excess of 10m which are suitable for 10-15 MW turbines. Wind turbine installation vessels need to be upgraded and new vessels may be required to install turbines with rotor diameters of over 220m. Nacelle weights are within the lifting capability of existing installation vessels, although leg and boom extensions may be required to reach the 150m or more hub height of these machines.

Gearing up to 2022/23

The significant increase in offshore wind turbine size will reduce the levelized cost of energy (LCoE) below 50 EUR/MWh for bottom-fixed offshore wind farms. The supply chain is gearing up and will be ready to build projects utilizing new turbine technology. The next generation of fixed foundation offshore wind turbines will also accelerate the commercialization and widescale deployment of floating wind through cost reductions in floating support structures and mooring systems.

The offshore wind industry and turbine manufacturers have continued with the evolutionary approach to scale up proven wind turbine technology. If done right, this approach will enable new wind farms to use the next generation of 10-15 MW wind turbines, benefiting from the reduction in LCoE, while managing risks and reducing the cost of finance.

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